Current color scheme was applied upon color scheme selection dialog
display because the GtkTreeView::cursor-changed signal is emitted when
the tree view is initially shown (since it actually gets the focus for
the first time), uselessly re-applying the current color scheme.

This is a performance issue because when many documents are open
updating the color scheme can take a few seconds.

Now we watch for the GtkTreeSelection::changed signal on the tree
view's selection, which is only emitted when the selected item actually
changes.  We also connect to that signal after filling the tree so we
don't get notified on the initial selection setup.